Silver halide photography usually involves the exposure of silver halide with light in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum. Thus, when silver halide is to be exposed to other wavelengths of radiation, such as green or red light in a multicolor element or infrared radiation in an infrared-sensitive element, a spectral sensitizing dye is required. Sensitizing dyes are chromophoric compounds (usually cyanine dye compounds) that are adsorbed to the silver halide. They absorb light or radiation of a particular wavelength and transfer the energy to the silver halide to form the latent image, thus effectively rendering the silver halide sensitive to radiation of a wavelength other than the blue intrinsic sensitivity. Sensitizing dyes can also be used to augment the sensitivity of silver halide in the blue region of the spectrum.
Spectral sensitizing dyes such as cyanine dyes are often used as combinations of dyes to achieve varying effects. For example, combinations of dyes can be used to provide emulsions with spectral sensitivity curves (a plot of sensitivity versus wavelength of exposure) that could not be easily obtained with a single dye. In other cases, a combination of dyes can be used to sensitize an emulsion to a greater degree than possible with either of the dyes alone or even greater than the predicted additive effect of the dyes. This phenomenon is known as supersensitization. Supersensitization and supersensitizing dye combinations have been widely discussed in the art. See, for example, P. Gilman, Review of the Mechanisms of Supersensitization, Photographic Science and Engrg., 18, pp. 418-430, July/August, 1974, T. Penner & P. Gilman, Spectral Shifts and Physical Layering of Sensitizing Dye Combinations in Silver Halide Emulsions, Photographic Science and Engrg., 20, pp. 97-106, May/June, 1976, and James, The Theory Of the Photographic Process 4th, pp. 259-265, 1977.
U.S. Pat. No. 3,527,641 of Nakazawa et al describes supersensitizing combinations of trimethine cyanine dyes. The supersensitizing effect is purportedly achieved by manipulation of the back ring substituents on the heterocyclic rings of these dyes, with a general teaching that essentially any known substituent may be utilized as the nitrogen substituent on these dyes. Such an approach does nothing, however, to alleviate the problem of retained dye stain.
During processing of color photographic materials, the silver halide is removed from the material. With black and white materials, the silver halide that was not exposed is removed. In either case, it is desirable to remove the sensitizing dye as well. Sensitizing dye that is not removed tends to cause retained dye stain, which adversely affects the image recorded in the photographic material. The problem of retained sensitizing dye stain is further aggravated by the increasing use of tabular grain emulsions and high chloride emulsions. Tabular grain emulsions have a high surface area per mole of silver, which can lead to higher levels of sensitizing dye and thus, higher levels of retained sensitizing dye stain. High chloride emulsions necessitate the use of sensitizing dyes having enhanced adsorption to silver halide, which can also lead to higher levels of dye stain. High chloride emulsions are also often subjected to rapid processing, which can aggravate dye stain problems.
It is thus an object of this invention to provide effective supersensitizing dye combinations of photographic sensitizers that also exhibit comparatively low dye stain.